The present embodiment relates to methods and compositions for treating subterranean zones in formations penetrated by well bores utilizing strongly delayed polymer breakers.
Treating fluids containing polymer breakers are used in a variety of operations and treatments in oil and gas wells. An example of a well completion treatment which utilizes a polymer breaker in a high viscosity fluid is known in the art as gravel packing. In gravel packing treatments, solid gravel particles such as sand are carried by way of the well bore to a subterranean zone in which a gravel pack is to be placed by a viscous gelled carrier fluid. That is, particulate solids (referred to in the art as gravel) are suspended in the high viscosity carrier fluid at the surface and carried to the subterranean zone in which the gravel pack is to be placed. Once the gravel is placed in the zone, the viscous carrier fluid is broken (the viscosity is reduced) and recovered (returned to the surface) by including a delayed polymer breaker, i.e., a viscosity reducing agent, in the carrier fluid. The gravel pack produced functions as a filter to separate formation solids from produced fluids while permitting the produced fluids to flow into and through the well bore.
In open hole gravel packing procedures, a non-viscous carrier fluid can be used that includes a polymer breaker which breaks down drill-in fluid filter cake left on the walls of the open hole well bore from the well bore drilling operation. The carrier fluid for open hole gravel packing can also be viscosified. In that case, the delayed breaker in the carrier fluid breaks the carrier fluid and the filter cake so that the carrier fluid and the filter cake can be removed from the subterranean zone.
The well completion procedures utilizing polymer breakers can be improved if the polymer breakers have a delayed reaction on the viscosity of the treatment fluid or on the degradation of the filter cake. For example, breaker compositions that include sodium persulfate and lithium hypochlorite which generally provide delayed breaks in the range of 0 to 2 hours are utilized in these operations. Recently, however, it has been recognized that even greater improvements to and simplification of well completion procedures can be realized if the breaks in viscosity of a carrier fluid or filter cake integrity can be even more strongly delayed. In this context and as used herein, the term “strongly delayed” as used in connection with a break in viscosity of a carrier fluid or filter cake integrity means a break delay of more than 3 hours.
In well temperatures above 150° F., t-butyl hydroperoxide has been found to function as a strongly delayed breaker. However, in well temperatures below 150° F., it has proven to be difficult to obtain strongly delayed, controllable break times of biopolymer components such as xanthan and succinoglycan gums of viscosified fluids or filter cakes. Attempts to obtain strongly delayed, controllable break times by reducing the concentration of the breaker generally results in incomplete breaks of the polymer and may be damaging to the permeability of the producing zone.
Thus, there is a need for treating fluid breaker systems which can provide controllable, strongly delayed breaks of biopolymer viscosified aqueous well treating fluids and filter cakes at temperatures ranging from 80 to 150° F.